1. Field of the Invention
This invention relates to an apparatus for transferring semiconductor substrates.
2. Description of the Related Art
Semiconductor substrates are normally housed in a substrate container such as wafer cassettes or wafer carriers when conveyed from a work station to another in a manufacturing line. Such containers are normally made of plastic material or a similar lightweight and low cost material.
On the other hand, plastic containers of the above described type cannot, however, be used when substrates are heat treated on a batch basis and, therefore, if such is the case, they are replaced by a substrate holder such as wafer boats as they are usually called, made of a material which is resistive to heat and corrosion, chemically stable and less liable to produce dusts such as quartz.
Normally, a particular apparatus is installed to automatically transfer semiconductor substrates from a substrate container to a substrate holder. Such a transferring apparatus typically comprises a number of arms for respectively supporting substrates, a drive means for driving the arms to operate and a rotating means for changing the direction to which the transferring apparatus is oriented. The arms of a transferring apparatus under consideration may be of the type that hold substrates by vacuum as disclosed in Published Unexamined Japanese Patent Application (JP-A) No. 2-71544 or the type that dispose substrates on respective supporting tables as disclosed in Published Examined Japanese Patent Application (JP-B) No. 2-39009 and JP-A-64-6047.
While the arm of the vacuum type is capable of securely holding a substrate, it can also gather and suck dusts from the surroundings, which by turn adhere to the substrate to consequently reduce the yield of manufacturing semiconductor devices.
With the arm of the supporting table type as disclosed in JP-B-2-39009, on the other hand, in a case that the substrate is charged, the entire surface of the substrate sustained on the table is held in contact with the surface of the wafer supporting table to generate static electricity between them that causes the substrate and the table to attract or repel each other. Consequently, the substrate is held on the supporting table under a rather unstable condition and can eventually drop from the table or generate dusts as they are scratched against each other. The generated dusts can by turn adhere to the substrate to consequently reduce the yield of manufacturing semiconductor devices as in the case of the vacuum type.